December 4, 2008

Insects: Out of Sight, Not Out of Mind

by Sarah Benton,
former Science Center Naturalist

One of the joys of winter is that we are seldom troubled by insects during our outdoor excursions. Although most insects are not active during the winter there are signs that they are still among us. An easily observed sign of insects in winter are galls. A gall is a benign growth on a plant leaf, stem or branch initiated when an insect lays its eggs on the plant. Two of the most common types of galls seen in winter are found on the stems of Goldenrod plants. The elliptical goldenrod gall is an elongated growth caused by the larva of a moth, Gnorimoschema gallaesolidaginis or Solidago Gall-moth, which is unusual as few moths cause galls. Eggs are laid on stems and over winter there. In the spring the larva hatch and move their way toward new goldenrod shoots, burrowing into the end buds and traveling down into the stem. The larva stops to feed and the plant either produces new cells or enlarges existing cells to form a gall around the larva. It remains in the gall feeding through late July when it bores an exit hole, then retreats back to its chamber to pupate, emerging in August or September as an adult. These elliptical galls are empty of the moth during the winter; however evidence of parasites of these galls such as Ichneumon wasp cocoons may be visible during this time of year.

Another easily located gall is called the Goldenrod ball gall. This gall, as the name indicates, is a spherical shape and is caused by the larva of the spotted-winged fly, Eurosta solidaginis. The eggs are laid on the stem of the Goldenrod plant in late May and June. The eggs hatch and the larva proceeds to burrow into the stem, where it hollows out a chamber for itself. The gall forms around this chamber and here the larva remains through the winter. In the spring the larva chews an exit tunnel to the outside, retreats to the inner chamber to pupate, and finally the adult fly emerges from the tunnel. The entire lifecycle of this insect can take place on one plant!

Galls are not the only signs left by insects in the winter. Next time you are out for a winter walk see if you can detect any other signs of insect life. They might not be out flying, buzzing, or biting yet, but they have not disappeared for good!

1 comment:

Anonymous said...

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